An attempt was made to assess the effects of increased body concentration of DA on the activities of catecholamine neurons in the periphery and brain. This was achieved by following the metabolism of ingested deuterated L-DOPA. It is assumed that after the ingestion of deuterated DOPA the excretion of deuterated dopamine and its metabolite will closely mimic the fate of ingested L-DOPA while changes in non-deuterated dopamine, norepinephrine and their metabolites will reflect upon the effects of increased body DA from ingested L-DOPA on peripheral and central catecholamine neurons. Since L-DOPA is commonly coadministered with peripheral dopadecarboxylase inhibitors, the influence of two types of dopadecarboxylase inhibitors on the metabolism of L-DOPA were also evaluated. The inhibitors are carbidopa (a peripheral drug) and alphamethyldopa (a compound that easily crosses the blood brain barrier). These two inhibitors were selected because alphamethyldopa is a metabolite of carbidopa. The possibility exists that coadministration of carbidopa with L-DOPA may produce central effects that can be attributed to alphamethyldopa.